Flame ionization detector

ABSTRACT

A thermionic flame ionization detector, particularly for the detection of phosphoro-organic compounds and comprising a flame jet and two electrodes which are arranged such that the wanted ionic current is derived from between the two electrodes and the unwanted currents are derived between one electrode and the jet.

United States Patent Inventor Frank P. Speakman Cambridge, England Appl. No. 768,788 Filed Oct. 18, 1968 Patented Oct. 26, 1971 Assignee Pye Limited Cambridge, Mass. Priority Oct. 31, 1967 Great Britain 495 12/67 FLAME IONIZATION DETECTOR 4 Claims, 2 Drawing Figs.

[1.8. Ci. 23/254 E, 23/232 C, 23/255 E Int. Cl 60111 31/12 Field of Search. ....23/254, 254

E, 255 E, 232 C [56] References Cited UNITED STATES PATENTS 3,129,062 4/1964 Ongkiehong et al 23/232 C 3,372,000 3/1968 Gallaway et al. 23/254 E 3,423,181 H1969 Dimick etaL. 23/232EX 3,489,523 l/l970 Clardy et al. 23/254E Primary ExaminerMorris O. Wolk Assistant ExaminerR. M. Reese Attorney-Holcombe, Wetherill & Brisebois ABSTRACT: A thermionic flame ionization detector, particularly for the detection of phosphoro-ozrganic compounds and comprising a flame jet and two electrodes which are arranged such that the wanted ionic current is derived from between the two electrodes and the unwanted currents are derived between one electrode and the jet.

PATENTEDUET 2s IHYI SHEET 1 UF 2 FLAME IONIZATION DETECTOR The present invention relates to thermionic flame ionization detectors and more particularly to thermionic flame ionization detectors of the type used for the detection of phosphoro-organic compounds.

Many modern pesticides contain phosphorous and it is therefore important to be able to detect and assess extremely low levels of phosphorous compound residue in agricultural food crops. This is conveniently done by chromatographic separation of materials extracted by solvents from the crops. Detection of the separated components is normally carried out using a thermionic flame ionization detector whose response to phosphorous containing compounds is enhanced by seeding the flame with alkali halides, such as the chloride, bromide and sulfate salts of sodium, caesium, potassium and rubidium.

In a detector of this type the signal current comprises three principal components. namely:

1. Current due to the passage through the detector of phosphoro-organic compounds-1 2. Current due to the passage of organic compounds-l 3. Current due to the seeding of the flame with the alkali ha- For maximum detector sensitivity to phosphoro-organic compounds and for maximum selectivity, 1,, needs to be large compared with I step, I However, in existing detectors 1,, is related to 1,, and although an increase in the seeding of the flame gives rise to an increased response to phosphoro-organic compounds it also causes an approximately prorata increase of standing current I It is thus not possible to increase the sensitivity of known detectors beyond a certain limit by increasing the amount by which the flame is seeded, all three currents 1 I and I flowing in the measuring circuit.

The object of the present invention is to provide a novel construction of detector in which this short-coming is at least partially overcome.

According to the present invention, a flame ionization detector is so constructed that the wanted and unwanted ionic currents are caused to flow, at least substantially, in separate external circuits.

The desired result may be achieved by an arrangement of the electrodes of the detector and according to one form of the invention two coaxial electrodes are provided which are arranged to receive ionic currents from the flame jet. in operation the two electrodes are connected to separate circuits such that one circuit receives most of the ionic current due to any phosphoro-organic compound present in the detector, while the second circuit receives most of the unwanted current due to the combustion of organic compounds and to seeding of the flame.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1, is a longitudinal cross section of one embodiment of detector according to the present invention, and

FIG. 2, shows partly diagrammatically the use of the detector of F IG. 1 in a chromatographic system.

Referring to FIG. 1, the embodiment of thermionic flame ionization detector to be described comprises a body member 1 of stainless steel containing a flame jet 2 and outer and inner electrodes 3 and 4 mounted coaxially within the detector and insulated therefrom by means of insulating mounts 5 and 6 respectively. The flame jet 2 is electrically connected to body 1 by virtue of its mounting and is surrounded by a sintered disc 2a to minimize turbulence in the region of the flame jet, due to the air supplied to the detector. Outer electrode 3 which is of open cylindrical form and has a perforated wall, comprises a removable electrode tip 7 which extends slightly beyond the end of electrode 4 and carries on its inner side an annular band 8 of caesium bromide flame seeding material. Bore 11 in body 1 couples jet 2 to gland l2 and bores 13 and 14 extend through the body wall to bore H and to the region surrounding the flame jet respectively.

Electrical connections to inner and outer electrodes 3 and 4 are made by means of insulated cables 9 and respectively,

and bore 14a couples the chamber of the detector body to the outer atmosphere.

Referring to FIG. 2, the detector according to the invention is shown in conjunction with associated equipment such as will be used for the detection and recording of separated components emerging from gas chromatographic column 18. The input circuit of a DC amplifier 15 is connected in series with polarizing source R6, of the order of 170 volts, between the electrodes 3 and 4 of the detector and the same polarizing source is also connected in series between electrode 3 and flame jet 2, the latter being earthed.

The output of the amplifier is connected to indicating or recording means 117, which may be a conventional pen recorder.

in operation, components of the sample under separation in the column 18 emerge at different times and are fed through the bore 11 to the detector flame jet where they are burnt in a hydrogen flame. Hydrogen is introduced to the flame jet through bore 13, combustion being supported by air introduced under pressure through bore 14. The air also serves to maintain a movement of combustion products through the detector towards and out of bore 14a, to minimize deposition upon the electrodes and insulating surfaces.

Some of the separated components in the detector leads to production of ions which gives rise to ionic currents through the two electrical circuits. Due to differences in mobility and life of the various ions formed in the Iilame and due to space charge effects, the circuit including the amplifier input carries preferentially most the current 1, due to any phosphoro-organic compound present in the detector and the second circuit, that between flame jet 2 and electrode 3, carries preferentially most of the current due to the combustion of organic compounds and due to seeding of the flame. Flow of current through amplifier input circuit 15 will give rise to a corresponding amplified output signal, which is used to produce a trace upon the chart of the pen recorder 17, the amplitude of peaks thereon being indicative of the amount of specific phosphoro-organic compounds present in the separated solvent.

Known thermionic flame ionization detectors seeded in this way to enhance their sensitivity to phosphoro-organic compounds have a selectivity of the order of l,000:l compared to a hydrocarbon while the detector herein described has a selectivity of the order of 1,000,000:0 compared to a hydrocarbon.

lclaim:

l. A thermionic flame ionization detection arrangement, comprising:

a detector having a body member containing a flame jet electrode,

inner and outer electrodes mounted in the body member coaxially one within the other,

said jet electrode being coaxial with the inner electrode,

said outer electrode extending beyond the end of the inner electrode in the direction of the flame jet electrode,

a removable electrode tip carrying a flame seeding material mounted on said outer electrode adjacent the jet electrode,

means for applying a polarizing potential between said inner and outer electrodes,

a first circuit path whereby a first ionic current representative of a wanted component can be derived from between said inner and outer electrodes,

a second circuit path in parallel with said first circuit path whereby a second ionic current representative of an unwanted component can started, derived between said outer electrode and said jet electrode,

and means for detecting said first ionic current representative of a wanted component.

2. A thermionic flame ionization detection arrangement for the detection of phosphoro-organic compounds, comprising:

a detector having a body member containing a flame jet electrode, introduced inner and outer electrodes mounted in the body member coaxially one within the other,

said jet electrode being coaxial with the inner electrode,

said outer electrode extending beyond the end of the inner electrode in the direction of the flame jet electrode,

a removable electrode tip carrying a flame seeding material mounted on said outer electrode adjacent the jet electrode,

a source of polarizing potential connected between said inner and outer electrodes,

a first circuit path including said source of polarizing potential whereby a first ionic current representative of a wanted component can be derived from between said inner and outer electrodes,

a second circuit path in parallel with said first circuit path and including said source of polarizing potential whereby a second ionic current representative of unwanted components can be derived between said outer electrode and said jet electrode and means connected in said first circuit path for detecting said first ionic current representative of a wanted component.

3. An arrangement as claimed in claim 2, in which said detecting means for said first ionic current comprises a DC amplifier connected in series with said polarizing source between said inner and outer electrodes and indicating or recording means connected to the output of said DC amplifier.

4. A detector as claimed in claim 2, in which said body member is made of metal, said jet electrode is electrically connected to said body member and said inner and outer electrodes are electrically insulated from said body member. 

2. A thermionic flame ionization detection arrangement for the detection of phosphoro-organic compounds, comprising: a detector having a body member containing a flame jet electrode, introduced inner and outer electrodes mounted in the body member coaxially one within the other, said jet electrode being coaxial with the inner electrode, said outer electrode extending beyond the end of the inner electrode in the direction of the flame jet electrode, a removable electrode tip carrying a flame seeding material mounted on said outer electrode adjacent the jet electrode, a source of polarizing potential connected between said inner and outer electrodes, a first circuit path including said source of polarizing potential whereby a first ionic current representative of a wanted component can be derived from between said inner and outer electrodes, a second circuit path in parallel with said first circuit path and including said source of polarizing potential whereby a second ionic current representative of unwanted components can be derived between said outer electrode and said jet electrode and means connected in said first circuit path for detecting said first ionic current representative of a wanted component.
 3. An arrangement as claimed in claim 2, in which said detecting means for said first ionic current comprises a DC amplifier connected in series with said polarizing source between said inner and outer electrodes and indicating or recording means connected to the output of said DC amplifier.
 4. A detector as claimed in claim 2, in which said body member is made of metal, said jet electrode is electrically connected to said body member and said inner and outer electrodes are electrically insulated from said body member. 